Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
London’s tone is contemplative—he often steps away from plot to comment on the way Buck's learning, how Buck’s character changes, or what the call of the wild surroundings begins to mean to Buck.
These contemplative passages are most striking at certain key events in the story, such as when Buck chases the rabbit in Chapter Three or, of course, at the end, when we see Buck in his new role in the wild:
On the opposite slope of the watershed they came down into a level country where were great stretches of forest and many streams, and through these great stretches they ran steadily, hour after hour, the sun rising higher and the day growing warmer. Buck was wildly glad. He knew he was at last answering the call, running by the side of his wood brother toward the place from where the call surely came. Old memories were coming upon him fast, and he was stirring to them as of old he stirred to the realities of which they were the shadows. He had done this thing before, somewhere in that other and dimly remembered world, and he was doing it again, now, running free in the open, the unpacked earth underfoot, the wide sky overhead. (7.16)
The author is also clearly sympathetic to Buck’s character, rendering him in a genuine and compassionate manner. Who's a good dog?